Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired hemolytic anemia associated with a bone marrow defect in which erythrocytes develop an abnormal sensitivity to lysis by complement. The number of erythroid and myeloid progenitors, as detected by culture in a methylcellulose colony assay, were significantly decreased in bone marrow and blood of PHN patients. A higher proportion of the most primitive progenitors (BFU-E) were in cell cycle in PNH patients than normal individuals or other hemolytic anemias, which may be a result of increased recruitment due to destruction of maturing cells. To identify at what stage during differentiation cells became susceptible to lysis, progenitors and their in vitro progeny were incubated under conditions that promoted complement-mediated lysis. The progenitors were insensitive to lysis; the progeny were variably sensitive to lysis, indicating that the susceptibility of cells to lysis occurred late during erythropoiesis. In addition, we found that the proportion of abnormal cells within each erythroid colony did not fit the expected result of two separate populations of progenitors giving rise to either completely defective or normal progeny. To further test the hypothesis that one population of progenitors exists in the PNH bone marrow, we used a polyclonal antiserum to a cell surface protein missing on defective cells. This functionally important molecule, called decay accelerating factor (DAF), regulates the deposition of the complement component C3b on the cell membrane. Using flow microfluorometry to measure DAF expression, two populations of red blood cells (rbc's) were detected in PNH patients, one corresponding to the negative control and the other to the distribution of DAF on normal rbc's. The negative population was reduced when cells were incubated with acidified serum prior to analysis. DAF expression was measured on erythroid progenitors and their in vitro normoblast progeny. In parallel with the previous studies, DAF was found present on the progenitors (BFU-E and CFU-E) and mostly absent on the in vitro normoblast progeny.